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If a function does not throw, return the function return value; otherwise, return the return value of a second function.


npm install @stdlib/utils-try-then


var trythen = require( '@stdlib/utils-try-then' );

trythen( x, y )

If a function x does not throw, returns the return value of x; otherwise, returns the return value of a second function y.

function x1() {
    return 1.0;

function x2() {
    throw new Error( 'beep' );

function y() {
    return -1.0;

var z = trythen( x1, y );
// returns 1.0

z = trythen( x2, y );
// returns -1.0

The function y is provided a single argument:

  • error: the error thrown by x
var randu = require( '@stdlib/random-base-randu' );

function x() {
    if ( randu() < 0.5 ) {
        throw new Error( 'beep' );
    throw new TypeError( 'boop' );

function y( err ) {
    if ( err instanceof TypeError ) {
        return 'boops';
    return 'beeps';

var z = trythen( x, y );
// returns <string>


var randu = require( '@stdlib/random-base-randu' );
var ceil = require( '@stdlib/math-base-special-ceil' );
var repeatString = require( '@stdlib/string-repeat' );
var trythen = require( '@stdlib/utils-try-then' );

var z;
var i;

function x() {
    if ( randu() < 0.9 ) {
        throw new Error( 'BOOP' );
    return repeatString( 'BOOP', ceil( randu()*3.0 ) );

function y() {
    return repeatString( 'beep', ceil( randu()*5.0 ) );

for ( i = 0; i < 100; i++ ) {
    z = trythen( x, y );
    console.log( z );

See Also

  • @stdlib/utils-try-catch: if a function does not throw, return the function return value; otherwise, return y.
  • @stdlib/utils-async/try-then: if a function does not return an error, invoke a callback with the function result; otherwise, invoke a second function.


This package is part of stdlib, a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop stdlib, see the main project repository.






Copyright © 2016-2024. The Stdlib Authors.

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  • stdlib-bot
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